In the case of a vehicle provided with an automatic transmission system controlled in response to the amount of operation of an accelerator pedal, the control of the vehicle during driving may become difficult when the actual acceleration signal representing the amount of operation of the accelerator pedal at each instant cannot be obtained correctly because of trouble with an acceleration sensor or the like. To eliminate this problem, there has been proposed a backup system in which a signal representing a predetermined fixed amount of operation of the accelerator pedal is used instead of the actual acceleration signal when trouble arises with the acceleration sensor (Japanese Patent Application Public Disclosure No. Sho 60-75735).
The backup signal of the proposed system is of fixed value and there is thus no problem in the case where the vehicle continues to run at a given speed. However, it becomes impossible to stop the vehicle, even when the brake pedal is depressed by the driver. The engine speed may become extremely high when the transmission is shifted to neutral. Furthermore, when, for example, the backup acceleration signal is used in place of the acceleration signal when the vehicle is stopped, a very dangerous condition may arise in which the vehicle will start independently of the driver's intention.